1. Field of the Invention
The present invention relates to the mounting of an encoder for the attainment of measurement accuracy and to the registration of color images in a color image output terminal. More particularly, the invention relates to antirotation encoder interfaces of several mechanisms used for registration of color images.
2. Description of the Related Art
Image registration is an important and difficult problem in a xerographic type color image output terminal. In FIG. 1, a color image output terminal 10 is shown having four photoreceptors 12, 14, 16 and 18. Each photoreceptor carries a unique color separation obtained by a conventional xerographic processor having charge device 20, write device 22 and develop device 24. The four color separations are transferred to intermediate belt 26 so as to coincide with one another and produce a full color image 1. Subsequently, the color image is transferred to paper 6 and the color image is fixed thereon. Photoreceptors 12, 14, 16 and 18 contain rotating members 1, 2, 3 and 4 respectively. Intermediate belt 26 is driven by rotating member 5.
In order to deliver good quality images, strict specifications are imposed on the accuracy with which the color image output terminal 10 superimposes the various color separations which compose the individual images. This juxtaposition accuracy is often called registration. In the trade, a limit of 125 micrometers is considered a maximum for acceptable misregistration errors of quality pictorial color images and a 75 micrometer limit is often imposed as a limit by the manufacturers of top quality equipment. These numbers represent the diameter of a circle which would encompass all supposedly homologous color dots.
In a single pass image output terminal, the various color separations are produced by separate imaging members and are passed to the intermediate belt where they are collected in juxtaposition. Registration errors can arise from motion errors of the collecting device and from mismatch of the individual separations.
With respect to the motion of the collecting device, good registration goals are attainable if the unit is designed such that its kinematic errors are made synchronous with the spacing distance between image transfer points from photoreceptors 12, 14, 16, and 18 to belt 26. In this manner, the modulation of the surface motion is repeatable (synchronous) with the imaging pitch and color on color separation errors are minimized. Although the absolute position error of each color may be large, the relative position error between colors is held to specification. The absolution image distortion is usually tolerable.
With respect to the imaging modules, the distortion created in the color separations contribute to misregistration to the extent that they are mismatched. In tandem image output terminals, where the separations are generated and developed on individual photoreceptors and then transferred to an intermediate belt or to copy paper, a mismatch in the motion errors of the photoreceptors contributes to misregistration. In machine architectures where rotation of the photoreceptor supporting members 1, 2, 3, and 4 and belt drive member 5 are controlled by closed loop servos with feedback from encoders, the run out error of the encoder shaft (eccentricity between the encoder shaft and the roll centers of rotating members 1, 2, 3, 4 and 5) adds to the inherent encoder error and becomes a significant factor. In FIG. 2, motor-encoder pair 30 is shown having a motor 32 including an immovable member 34 and rotating member 36. Motor-encoder pair 30 also has encoder 40 including stator 42 and rotor 44. Rotor 44 is coupled with rotating member 36 through flexible coupler 46. Rotatably immovable member 34 and stator 42 are fixed to frame 38 (also referred to as a ground) so that the rotational axis of member 36 is substantially parallel to the rotational axis of rotor 44. Due to tolerances, these two rotational axes are not coincident therewith.
Flexible couplers such as shown in FIG. 2 enable the motor-encoder pair 30 to be produced at low cost and with relaxed tolerances on manufactured components since flexible coupler 46 yields to small misalignments. Under the influence of exciting torques, the compliance of flexible coupler 46 causes the rotation of rotor 44 to differ from that of rotating member 36 due to inertia of rotor 44. This difference limits the timely response of the encoder to changes in angular position of rotating member 36 and the dynamic mechanical resonance in the motor-encoder pair limits, in general, the performance of the servo control system.
If flexible coupler 46 were to be made of rigid material, the timely response of the motor-encoder pair would be improved; however, misalignments between the axis of rotor 44 and the axis of rotating member 36 would stress and quickly cause the failure of the bearings of either encoder 40 or motor 32. In order to avoid damage to the bearings when flexible coupler 46 is made rigid, it is necessary to provide a flexible mounting for either motor 32 or encoder 40 or both.
One such flexible mounting is shown in FIGS. 3A and 3B, wherein a shaft of motor 52 is rigidly coupled to a shaft of encoder 54 through rigid coupler 53. Antirotation arm 56 is fastened to a stator of encoder 54 by fastening means 58. In FIGS. 3A and 3B, the fastening means 58 is shown to be a pair of screws. The antirotation arm 56 is pinched between frame 62 (ground) and forcing device 64 which is backed by additional ground 66 so that antirotation arm 56 is slidable in a translation direction 68. The encoder shaft center B is in the center of the end view of encoder 54. Because of misalignment between motor 52 and encoder 54, encoder shaft center B rotates in a small circle 70 having a radius e due to the shaft eccentricity of either the encoder shaft or motor shaft or coupling 53. The encoder shaft center B is able to move in the translation direction 68 and in a direction transverse to the translation direction 68, and in the plane of FIG. 3B, due either to siding at the contact of the antirotation arm 56 with frame 62, or to pivoting of the antirotation arm 56 about the contact point with frame 62. In either case, C is the slide center of the antirotation arm, and B is the encoder shaft center defining length L therebetween.
Although the encoder interface shown in FIGS. 3A and 3B rigidly couples a shaft of the encoder 54 to a shaft of the motor 52, the interface causes an undesirable rotation of the housing of the encoder 54 due to eccentricity e between the shafts of the motor and the encoder. This undesired rotation causes errors in the readout of the encoder.
FIG. 4 shows a schematic of this error source. As encoder shaft center B rotates about the small circular orbit 70 having radius e due to the eccentricity between the two shafts, and having a center of encoding 74 the encoder housing 72 rotates by a small angle .beta. which varies according to the position of encoder shaft center B along the track of circular orbit 70. Thus, since the encoder produces a signal indicative of the angle of the encoder rotor relative to the encoder stator, the eccentricity of either the encoder shaft 71 or motor shaft causes the indicated angle to differ from to the angle of the motor shaft relative to the machine frame, which is the desired quantity to be measured. When the stator rotates as illustrated in FIG. 4, the encoder signal contains an error caused by the rotation of the encoder housing 72 through angle .beta. given by the formula: EQU .beta.=arcsin[(e/L) sin.theta.]
where .theta. is the input rotation of the encoding shaft which is rigidly coupled to the eccentric motor shaft. Therefore, .theta. represents the position of encoder shaft center B along the track of circular orbit 70.
It is seen that even when an encoder interface is rigidly coupled to the rotating member of a motor, where the encoder shaft is free to translate, errors in the encoder signal are produced. For example, with eccentricity e=0.005 inches and L=1.0 inches, the maximum .beta. of the encoder housing would be 0.2865 degrees. This error reflected at the surface of a one inch diameter roller would be about 0.005 inches. To minimize .beta., L should be large in proportion to the magnitude of the eccentricity e. Since eccentricity e is dependent on the manufacturing costs expended in making the parts, a large eccentricity e corresponding to a low cost part having minimum controls on tolerances, would require a larger antirotation arm length L. There comes a point where the antirotation arm length L is impractically large.